According to Dictionary.com, poplolly is an affectionate term, meaning something to the effect of "little darling." Their editors suggest it comes from the French word "poupelet," which I've been unable to find any further information on. I tried the OED, Etymonline, and the Babelfish translator. Googling only seems to bring up information about Poupelet as a surname.

I also found the Dictionary.com definition to be at odds with the definition from the OED, which defines a poplolly as a mistress. The latter definition seems to fit with the usage in the novel I'm reading, which is Suzanne Enoch's After the Kiss (p. 250).